Organic light-emitting diode (OLED) devices are attractive because of their low driving voltage, high luminance, wide-angle viewing, and capability for full-color flat emission displays. Tang et al. described a multilayer OLED device in their U.S. Pat. Nos. 4,769,292 and 4,885,211. Efficient white light producing OLED devices (white OLEDs) are considered a low cost alternative for several applications such as paper-thin light sources, backlights in LCD displays, automotive dome lights, and office lighting. As with any light-emitting device, white OLEDs should be bright and efficient in terms of power consumption, The preferred spectrum and color of a white OLED will depend on the application for which it is intended. For example, if a particular application requires light that is to be perceived as white without subsequent processing that alters the color perceived by a viewer, it is desirable that the light emitted by the white OLED have Commission International d'Eclairage (CIE) chromaticity coordinates of about (0.33, 0.33). For other applications, particularly applications in which the light emitted by the OLED is subjected to further processing light that alters its perceived color, it may be satisfactory or even desirable for the light that is emitted by the OLED to be off-white, for example bluish white, greenish white, yellowish white, or reddish white. Herein the term “white” or “substantially white” will be used broadly to mean light that is perceived as white or off-white. A white OLED will mean an OLED whose emission is white in this broad sense. In any event, in accordance with this disclosure, white light is that light which is perceived by a user as having a white color.
The following patents and publications disclose the preparation of organic OLED devices capable of emitting white light, comprising a hole-transporting layer and an organic luminescent layer, and interposed between a pair of electrodes. White OLEDs have been reported before by J. Shi (U.S. Pat. No. 5,683,823) wherein the luminescent layer includes red and blue light-emitting materials uniformly dispersed in a host emitting material. This device has good electroluminescent characteristics, but the concentrations of the red and blue dopants are very small, such as 0.12% and 0.25% of the host material. These concentrations are difficult to control during large-scale manufacturing. Sato et al. in JP 7-142169 disclose an OLED device, capable of emitting white light, made by forming a blue light-emitting layer adjacent to the hole-transporting layer and followed by a green light-emitting layer having a region containing a red fluorescent material. Kido et al., in Science, Vol. 267, P 1332 (1995) and in Applied Physics Letters Vol. 64, p. 815 (1994), report a white light producing OLED device. In this device three emitter layers with different carrier transport properties, individually emitting blue, green, or red light, are used to generate white light. Littman et al. in U.S. Pat. No. 5,405,709 disclose another white emitting device, which is capable of emitting white light in response to hole-electron recombination, and comprises a fluorescent in a visible light range from bluish green to red. Recently, Deshpande et al., in Applied Physics Letters, Vol. 75, p. 888 (1999), described a white OLED using red, blue, and green luminescent layers separated by a hole blocking layer due to specific energy relationship between the emission layer (EML) and electron-transporting layer (ETL). These device structures contain multiple layers and are difficult to manufacture.
White OLEDs can be used with color filters in full-color display devices. They can also be used with color filters in other multicolor, or functional-color display devices. White light producing OLED displays are easy to manufacture, and they produce reliable white light in each pixel of the displays. However, the color filters each transmit only about 30% of the original white light. Therefore, high luminance efficiency is required for the white OLEDs. Although the OLEDs are referred to as white and may appear white or off-white, for this application, the CIE coordinates of the light emitted by the OLED are less important than the requirement that the spectral components passed by each of the color filters be present with sufficient intensity in that light. It is also important that the color, after passage through a color filter, be appropriate for the intended application. For use in a full-color display, typical desired colors after passage through a red, green, or blue filter are, respectively, red with CIE coordinates of about (0.64, 0.36), green with CIE coordinates of about (0.29, 0.67), and blue with CIE coordinates of about (0.15, 0.19). The devices must also have good stability in long-term operation. Because the devices must be run at high luminance, this is a demanding requirement.
A problem in this application of white OLEDs is that the intensity of the white emission spectrum is frequently lower than desired. Also the stability of these devices can be lower.